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vk3pa
Deceased
Australia
1714 Posts |
 Posted - 08 Sep 2008 : 07:20:04 AM
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First of all, you must calculate the Velocity Factor of the cable that
you intend to use. I would cut a half wavelength of coax, using a higher
VF than I would expect for that coax. I think that .8 VF would be safe
for most, and if you're using good foam dielectric coax or hardline, I'd
start with .9VF. So.. a half wave at .8VF is 468/F * .8 which is 468/
1.850 = 253 * .8 = 245ft.
Now, get your Antenna Analyser.. Connect a T piece to the antenna
socket, with a 50 Ohm resistor across one input, and your 245ft of coax
to the other. Your 245ft is open circuit at the far end.
Find your 1:1SWR frequency nearest to top band. Let's say that it
measures 1:1 at 1.680 mHz.
You now know, that 245ft of this particular coax is a half wave (180
degrees) at 1.680mHz. This information allows us to calculate the VF as
follows. 468/ 1.68 mHz = 278.6ft, which is a 1/2 w/l on 1.680 mHz in
free space.
So.. 245 is to 278.6 as X is to 100, so X = 87.9% = Velocity factor of .879
You would expect to see this .879VF only in good quality foam dielectric
coax. You'd expect around .68VF for cheap RG58 and RG8.
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calculating the Velocity Factor of the coax cable
